Renal damage is not improved by blockade of endothelin receptors in primary renin-dependent hypertension

Distinct effects of bosentan on NO-dependent vasodilation and calcium influx in heterozygous Ren-2 transgenic rats on high-salt diet

Our studies in hypertensive Ren-2 transgenic rats (TGR) demonstrated that chronic administration of atrasentan (ETA receptor antagonist) decreased blood pressure by reduced Ca2+ influx through L-type voltage-dependent calcium channels (L-VDCC) and attenuated angiotensin II-dependent vasoconstriction. We were interested whether bosentan (nonselective ET(A)/ET(B) receptor antagonist) would have similar effects. Young 4-week-old (preventive study) and adult 8-week-old (therapeutic study) heterozygous TGR and their normotensive Hannover Sprague-Dawley (HanSD) controls were fed normal-salt (NS, 0.6 % NaCl) or high-salt (HS, 2 % NaCl) diet for 8 weeks. An additional group of TGR fed HS was treated with bosentan (100 mg/kg/day). Bosentan had no effect on BP of TGR fed high-salt diet in both the preventive and therapeutic studies. There was no difference in the contribution of angiotensin II-dependent and sympathetic vasoconstriction in bosentan-treated TGR compared to untreated TGR under the condition of high-salt intake. However, bosentan significantly reduced NO-dependent vasodilation and nifedipine-sensitive BP component in TGR on HS diet. A highly important correlation of nifedipine-induced BP change and the BP after L-NAME administration was demonstrated. Although bosentan did not result in any blood pressure lowering effects, it substantially influenced NO-dependent vasodilation and calcium influx through L-VDCC in the heterozygous TGR fed HS diet. A significant correlation of nifedipine-induced BP change and the BP after L-NAME administration suggests an important role of nitric oxide in the closure of L-type voltage dependent calcium channels.

Renoprotective effects of ET(A) receptor antagonists therapy in experimental non-diabetic chronic kidney disease: Is there still hope for the future?

Chronic kidney disease (CKD) is a life-threatening disease arising as a frequent complication of diabetes, obesity and hypertension. Since it is typically undetected for long periods, it often progresses to end-stage renal disease. CKD is characterized by the development of progressive glomerulosclerosis, interstitial fibrosis and tubular atrophy along with a decreased glomerular filtration rate. This is associated with podocyte injury and a progressive rise in proteinuria. As endothelin-1 (ET-1) through the activation of endothelin receptor type A (ET(A)) promotes renal cell injury, inflammation, and fibrosis which finally lead to proteinuria, it is not surprising that ET(A) receptors antagonists have been proven to have beneficial renoprotective effects in both experimental and clinical studies in diabetic and non-diabetic CKD. Unfortunately, fluid retention encountered in large clinical trials in diabetic CKD led to the termination of these studies. Therefore, several advances, including the synthesis of new antagonists with enhanced pharmacological activity, the use of lower doses of ET antagonists, the addition of diuretics, plus simply searching for distinct pathological states to be treated, are promising targets for future experimental studies. In support of these approaches, our group demonstrated in adult subtotally nephrectomized Ren-2 transgenic rats that the addition of a diuretic on top of renin-angiotensin and ET(A) blockade led to a further decrease of proteinuria. This effect was independent of blood pressure which was normalized in all treated groups. Recent data in non-diabetic CKD, therefore, indicate a new potential for ET(A) antagonists, at least under certain pathological conditions.

Regulation of podoplanin expression by microRNA-29b associates with its antiapoptotic effect in angiotensin II-induced injury of human podocytes

BACKGROUND

Angiotensin (Ang)II is involved in induction of proteinuria, renal injury, and apoptosis and thus a major contributor to the development of chronic kidney disease. Podocytes are of major importance for the pathogenesis of several kidney diseases. Decrease of podoplanin (PDPN) in podocytes and podocyte loss has been associated with the development of proteinuria. Little is known about the regulation and biological function of PDPN in podocytes and its role in AngII-mediated kidney damage. Here, we determined the influence of AngII on the expression of PDPN, microRNA (miRNA)-29b and miRNA-497 in human podocytes. Further, we analyzed the impact of small interfering RNA-mediated downregulation of PDPN on AngII-induced apoptosis and viability. Moreover, we characterized the role of miRNA-29b and miRNA-497 in expression regulation of PDPN.

METHODS

Cell viability and apoptosis were determined by functional assays. Expression analyses were done via Real-Time PCR and western blot analyses. Dual luciferase assay was performed to characterize miRNA-mediated expression control.

RESULTS

AngII increased the expression of miRNA-29b and reduced PDPN. Small interfering RNA-mediated downregulation of PDPN increased proapoptotic caspase-3 activation and cytochrome C translocation, whereas cell viability and Akt phosphorylation were reduced in AngII-stimulated podocytes. In contrast to miRNA-497, transfection of cells with miRNA-29b mimics significantly decreased PDPN. Cotransfection of cells with miRNA-29b and a dual luciferase reporter vector decreased the luciferase activity compared with controls.

CONCLUSION

These data demonstrate the posttranscriptional control of PDPN expression by miRNA-29b and support a role of PDPN as an antiapoptotic prosurvival factor in AngII-induced injury of human podocytes.

Endothelin-1 Drives Epithelial-Mesenchymal Transition in Hypertensive Nephroangiosclerosis

BACKGROUND

Tubulointerstitial fibrosis, the final outcome of most kidney diseases, involves activation of epithelial mesenchymal transition (EMT). Endothelin-1 (ET-1) activates EMT in cancer cells, but it is not known whether it drives EMT in the kidney. We therefore tested the hypothesis that tubulointerstitial fibrosis involves EMT driven by ET-1.

METHODS AND RESULTS

Transgenic TG[mRen2]27 (TGRen2) rats developing fulminant angiotensin II-dependent hypertension with prominent cardiovascular and renal damage were submitted to drug treatments targeted to ET-1 and/or angiotensin II receptor or left untreated (controls). Expressional changes of E-cadherin and α-smooth muscle actin (αSMA) were examined as markers of renal EMT. In human kidney HK-2 proximal tubular cells expressing the ETB receptor subtype, the effects of ET-1 with or without ET-1 antagonists were also investigated. The occurrence of renal fibrosis was associated with EMT in control TGRen2 rats, as evidenced by decreased E-cadherin and increased αSMA expression. Irbesartan and the mixed ET-1 receptor antagonist bosentan prevented these changes in a blood pressure-independent fashion (P < 0.001 for both versus controls). In HK-2 cells ET-1 blunted E-cadherin expression, increased αSMA expression (both P < 0.01), collagen synthesis, and metalloproteinase activity (P < 0.005, all versus untreated cells). All changes were prevented by the selective ETB receptor antagonist BQ-788. Evidence for involvement of the Rho-kinase signaling pathway and dephosphorylation of Yes-associated protein in EMT was also found.

CONCLUSIONS

In angiotensin II-dependent hypertension, ET-1 acting via ETB receptors and the Rho-kinase and Yes-associated protein induces EMT and thereby renal fibrosis.

Contrasting effects of intervention with ETA and ETB receptor antagonists in hypertension induced by angiotensin II and high-salt diet

Endothelin (ET) receptor antagonists are antihypertensive and renoprotective in angiotensin II (AngII)-induced hypertension if administered when AngII infusion commences, but their effects on established hypertension are poorly understood. We therefore tested the effects of intervening with an ETA (ABT-627) or ETB (A-192621) receptor antagonist after establishing hypertension with AngII (65 ng/min s.c.) plus 8% NaCl diet (AngII-HS) in rats. Prior to administration of ABT-627, AngII-HS and AngII-HS plus ABT-627 groups displayed robust hypertension (mean arterial pressure (MAP), 170 +/- 5 and 165 +/- 5 mm Hg versus 110 +/- 3 mm Hg in normal salt control rats at day 7, P < 0.05). Administering ABT-627 from day 8 of AngII-HS treatment prevented further rises in MAP (168 +/- 5 and 191 +/- 3 mm Hg at day 13 in AngII-HS plus ABT-627 and AngII-HS, P < 0.001), without blunting the significant increases in urinary protein (19-fold), albumin (25-fold), or MCP-1 excretion (6- to 8-fold) or the reduction in creatinine clearance. Administering A-192621 from day 8 mildly exacerbated AngII-HS induced hypertension (P < 0.05 for AngII-HS versus AngII-HS plus A-192621 on days 11 and 12 only) and reduced plasma nitrite/nitrate concentration (P < 0.05), without affecting proteinuria, albuminuria, or creatinine clearance. These results confirm the importance of ETA receptor signaling in maintaining AngII-HS hypertension and suggest that including ETB receptor blockade in therapeutic approaches to treating hypertension would be ineffective or even counterproductive.

Endothelin receptor selectivity in chronic kidney disease: rationale and review of recent evidence

Endothelin (ET) is a potent vasoconstrictory peptide with proinflammatory and profibrotic properties that exerts its biological effects through two pharmacologically distinct receptor subtypes, namely ET(A) and ET(B). In addition to its substantial contribution to normal renal function, a large body of evidence suggests that derangement of the renal ET system is involved in the initiation and progression of chronic kidney disease (CKD) in diabetes, hypertension and glomerulonephritis. Thus, the use of ET receptor antagonists (ERAs) may offer potential novel treatment strategies in CKD. Recent literature on the role of the renal ET system in the healthy kidney was reviewed. In addition, an unbiased PubMed search was performed for studies published during the last 5 years that addressed the effects of ERAs in CKD. A particular objective was to extract information regarding whether selective or nonselective ERAs may have therapeutic potential in humans. ET-1 acts primarily as an autocrine or paracrine factor in the kidney. In normal physiology, ET-1 promotes diuresis and natriuresis by local production and action through ET(B) receptors in the renal medulla. In pathology, ET-1 mediates vasoconstriction, mesangial-cell proliferation, extracellular matrix production and inflammation, effects that are primarily conveyed by ET(A) receptors. Results obtained in animal models and in humans with the use of ERAs in CKD are encouraging; nevertheless, it is still under debate which receptor subtype should be targeted. According to most studies, selective inhibition of ET(A) receptors appears superior compared with nonselective ERAs because this approach does not interfere with the natriuretic, antihypertensive and ET clearance effects of ET(B) receptors. Although preliminary data in humans are promising, the potential role of ERAs in patients with CKD and the question of which receptor subtype should be targeted can only be clarified in randomized clinical trials.

Late-onset endothelin-A receptor blockade reduces podocyte injury in homozygous Ren-2 rats despite severe hypertension

We have recently found in male homozygous hypertensive Ren-2 transgenic rats (TGRs) fed a high-salt diet that early onset selective endothelin (ET) A (ET(A)) or nonselective ET(A)/ET B (ET(B)) receptor blockade improved survival rate and reduced proteinuria, glomerulosclerosis, and cardiac hypertrophy, whereas selective ET(A) receptor blockade also significantly attenuated the rise in blood pressure. Because antihypertensive therapy in general is known to be more efficient when started at early age, our study was performed to determine whether onset of ET receptor blockade at a later age in animals with established hypertension will have similar protective effects as does early-onset therapy. Male homozygous TGRs and age-matched normotensive Hannover Sprague-Dawley rats were fed a high-salt diet between days 51 and 90 of age. TGRs received vehicle (untreated), the selective ET(A) receptor blocker atrasentan (ABT-627), or the nonselective ET(A)/ET(B) receptor blocker bosentan. Survival rates in untreated and bosentan-treated TGRs were 50% and 64%, respectively, whereas with atrasentan, survival rate of TGR was 96%, thus, similar to 93% in Hannover Sprague-Dawley rats. From day 60 on, systolic blood pressure in atrasentan-treated TGRs was transiently lower (P<0.05) than in untreated or bosentan-treated TGRs. Glomerular podocyte injury was substantially reduced with atrasentan treatment independent of severe hypertension and strongly correlated with survival (P<0.001). Our data indicate that in homozygous TGR ET receptors play an important role also in established hypertension. Selective ET(A) receptor blockade not only reduces podocyte injury and end-organ damage but also improves growth and survival independently of hypertension.

Early-onset endothelin receptor blockade in hypertensive heterozygous Ren-2 rats

Male heterozygous Ren-2 transgenic rats and Hannover Sprague-Dawley rats fed a normal or high-salt diet were either untreated or treated with the nonselective receptor ET(A)/ET(B) receptor blocker bosentan or the selective ET(A) receptor blocker, ABT-627, known as atrasentan. Survival rate was partly increased by bosentan and fully normalized by atrasentan. Bosentan did not significantly influence the course of hypertension in TGR, whereas atrasentan significantly decreased BP on both diets. Atrasentan substantially reduced proteinuria, cardiac hypertrophy, glomerulosclerosis and left ventricular ET-1 tissue concentration on both diets. Our data indicate that ET(A) receptor blockade is superior to nonselective blockade in attenuating hypertension, end-organ damage and improving survival rate.

Modulation of osteopontin in proteinuria-induced renal interstitial fibrosis

Proteinuria is associated with macrophage-dependent interstitial fibrosis (IF). Osteopontin (OPN), a macrophage chemoattractant, may be involved in the transition of proteinuria to IF but protective properties have also been reported. To elucidate whether OPN may be involved in the proteinuria-induced cascade of tubulointerstitial damage, renal expression of OPN was studied during the development of proteinuria-induced renal damage and during anti-proteinuric intervention with ACE inhibition (ACEi). First, the temporal relationships between proteinuria, interstitial OPN induction, and IF in adriamycin nephrosis (AN), a model of chronic proteinuria-induced renal damage, were studied. Second, the effect of anti-proteinuric treatment on OPN expression was investigated. The time course of OPN induction and markers of renal damage was studied in rats with unilateral AN at 6-week intervals until week 30. In a second study, a renal biopsy was taken 6 weeks after induction of bilateral AN; subsequently, rats were treated with ACEi until termination (week 12). In unilateral AN, proteinuria developed gradually and stabilized at week 10. In proteinuric kidneys, OPN expression was induced from week 12 onwards. Simultaneously, a progressive increase in interstitial macrophages, alpha-smooth muscle actin (alpha-SMA), collagen type III, and focal glomerulosclerosis (FGS) was observed. In bilateral AN, ACEi reduced proteinuria and OPN protein and stabilized fibrosis. In untreated animals, OPN mRNA increased, with stable OPN protein and fibrosis and increased FGS. Thus, in AN, development of proteinuria is followed by up-regulation of OPN along with markers of renal damage. The up-regulation of OPN is reversible by anti-proteinuric treatment without a corresponding reduction in fibrosis. Whereas these data are consistent with a role for OPN in the cascade of transition from proteinuria to fibrosis, intervention with ACEi showed that reduction of OPN does not attenuate established fibrosis.

Expression and Response to Angiotensin-Converting Enzyme Inhibition of Matrix Metalloproteinases 2 and 9 in Renal Glomerular Damage in Young Transgenic Rats with Renin-Dependent Hypertension

Extracellular matrix expansion in the glomerular mesangium contributes to the development of glomerulosclerosis and chronic renal disease in arterial hypertension. Transforming growth factor-beta1 (TGF-beta1), matrix metalloproteinases (MMPs), and tissue inhibitors of MMPs (TIMPs) are involved in this process. Conflicting data are reported on the effects of angiotensin II (Ang II) and the response to angiotensin-converting enzyme inhibition on MMPs and TIMPs in early stages of hypertensive glomerular damage. We therefore investigated the effects of Ang II-dependent hypertension on MMP-2, MMP-9, TIMP-1, and TIMP-2 in isolated glomeruli of 8-week-old homozygous male rats overexpressing the mouse Ren2 gene [TGR(mRen2)27]. At this age, systolic blood pressure was already significantly elevated in Ren2 compared with Sprague-Dawley (SD) rats (197 +/- 38 versus 125 +/- 16 mm Hg, p < 0.01). Ren2 exhibited renal damage as determined by increased urinary albumin excretion, focal glomerulosclerosis, mesangial matrix expansion, and alpha-smooth muscle actin deposition. Quantification of mRNA levels in isolated glomeruli by real-time polymerase chain reaction showed a significant increase of TGF-beta1, a 2.3- and a 2.6-fold increase of MMP-2 and TIMP-1 in Ren2 compared with SD (p < 0.01, respectively) and no strain differences for TIMP-2. In contrast, MMP-9 mRNA expression was markedly suppressed to 10% of control levels in Ren2 (p < 0.01). Early treatment with ramipril completely prevented renal damage in Ren2 and restored mRNA expression of TGF-beta1, MMP-2, and TIMP-1 to SD control levels. Interestingly, down-regulation of MMP-9 mRNA, protein, and activity was not affected by ramipril, indicating that the protective effect of this compound is not attributable to restoration of MMP-9 in the glomerulus.

Early endothelin-A receptor blockade decreases blood pressure and ameliorates end-organ damage in homozygous Ren-2 rats

We have recently found that nonselective endothelin ETA/ETB receptor blockade markedly improves survival rate and ameliorates end-organ damage in male homozygous rats transgenic (TGR) for the mouse Ren-2 renin gene without lowering blood pressure. Because activation of the ETA receptor may be responsible for the detrimental effects of ET in the development of hypertension, this study was performed to determine whether ETA or ETA/ETB receptor blockade exerts these beneficial effects. TGR and age-matched normotensive Hannover Sprague-Dawley rats fed a high-salt diet received either vehicle or bosentan and atrasentan (ABT-627) as nonselective ETA/ETB and selective ETA receptor blockers, respectively, from 29 until 90 days of age. The survival rate of 48% in untreated TGR was significantly (P<0.01) improved to 79% by bosentan and to 92% by ABT-627 (ABT-627 versus bosentan P<0.05). Proteinuria, glomerulosclerosis, and cardiac hypertrophy, as well as ET-1 content in left ventricular tissue, were significantly reduced by bosentan and to a greater degree by ABT-627, which also significantly attenuated the rise in blood pressure (P<0.05). Our data indicate that the ET system, especially via ETA receptors, plays an important role in the development of hypertensive end-organ damage and confirm the concept that the predominant role of ETB receptors within the peripheral vasculature is to mediate the vasorelaxant actions of ET-1. They also demonstrate that selective blockade of ETA receptors is superior to nonselective ETA/ETB in attenuating hypertension, hypertensive organ damage, and survival rate.

Vascular endothelin in hypertension

Endothelins are powerful vasoconstrictor peptides that also play numerous other functions in many different organs. Endothelin-1 (ET-1) is the most abundant and important of this family of peptides in blood vessels. Production of ET-1 is increased in the endothelium and the kidney in salt-dependent models of hypertension (e.g.: DOCA-salt rats and Dahl salt-sensitive rats, in salt-loaded SHR-SP, in angiotensin II-infused and in diabetic rats). ET-1 elicits an inflammatory response by increasing oxidant stress in the vascular wall, which induces vascular remodeling and endothelial dysfunction found in the hypertensive models that exhibit an endothelin-mediated component. Endothelin receptor antagonism reduces blood pressure and vascular hypertrophic remodeling present in these hypertensive models. Patients with stage 2 hypertension have enhanced vascular expression of ET-1. Endothelin receptor antagonists lower blood pressure in hypertensive patients. They could become therapeutic agents for prevention of target organ damage in hypertension and in type 2 diabetes, chronic renal failure and congestive heart failure. Side effects of endothelin receptor blockers have prevented up to the present their development for these indications. New endothelin antagonists devoid of these side effects, or alternatively inhibitors of the endothelin converting enzymes that generate ET-1 may in the future become available to block the endothelin system. However, to date endothelin antagonists have been approved only for the treatment of primary pulmonary hypertension, a rapidly fatal condition in which the endothelin system plays an important role and endothelin antagonists exert favorable effects.